This invention relates to a photomask and to an exposure method using a photomask.
Semiconductor devices are becoming more and more tightly packed in density and increasingly fine, and this has been accompanied by the need to provide finer circuit patterns on semiconductor substrates. Various improvements in lithography have been made in order to satisfy this need.
In existing lithographic techniques, raising the fineness of resist patterns that are transferred to semiconductor substrates has been dealt with mainly by developing exposure equipment and especially by raising the numerical aperture (NA) of projection lens optics. In general, the limit on a fine pattern that is capable of being resolved (i.e., critical resolution R) and NA are related as follows: R=K1xc3x97xcex/NA (where K1 is a process-dependent constant such as the performance of a photosensitive resin and xcex represents wavelength). This means that the critical resolution can be reduced in inverse proportion to an increase in NA. However, depth of focus (DOF), namely the range over which displacement of focal point is allowed, and NA are related as follows: DOF=K2xc3x97xcex/NA (where K2 is a process-dependent constant). As a consequence, raising the numerical aperture NA has the converse effect of shortening depth of focus.
Currently existing semiconductor devices are manufactured by repeating the steps of film formation, resist-pattern formation and etching, etc. As a result, a step or difference in level on the order of several microns usually develops on the semiconductor substrate.
If it is attempted to form a resist pattern on a semiconductor substrate having such a step, the focal point at the top of the step will differ from that at the bottom, making it difficult to form a highly precise fine pattern. In view of this, obtaining a large depth of focus is a major challenge in the manufacture of modern semiconductor devices.
Studies for the purpose of increasing depth of focus have been conducted taking a variety of approaches. Specifically, one example of a technique employed on the side of the illumination optics is a super-resolution technique referred to as the modified illumination method or oblique incidence (off-axis) illumination method. One example of a technique which is an improvement on the side of the photomask is to use an auxiliary pattern. It should be noted that a photomask is a master sheet for exposure formed into a pattern comprising transparent and opaque areas and is referred to in particular as a xe2x80x9creticlexe2x80x9d unless the reduction ratio is 1:1. Here, however, the term xe2x80x9cphotomaskxe2x80x9d will be used regardless.
The modified illumination method, which approaches the problem of depth of focus from the side of the illumination optics, will be described in simple terms first. The modified illumination method is classified into a ring illumination method, which shields the central portion of an aperture stop and uses a ring-shaped illuminating light source, and a four-point illumination method that uses an aperture stop open only at the four corners of the sides thereof.
In an ordinary illumination optical system, light impinges upon the patterned photomask perpendicularly and transfers the pattern onto the semiconductor substrate through a projection lens system. Use is made of diffracted light of order 0 and order +1 or xe2x88x921 in pattern resolution. As the pattern becomes finer, however, the angle of diffraction increases and eventually only diffracted light of order 0 impinges upon the projection lens system. As a consequence, the light that passes through the fine pattern is substantially the perpendicular component only and there is a decline in the contrast of the light-intensity distribution on the image plane.
However, with the four-point illumination method, for example, only oblique light impinges upon the photomask. As a result, either diffracted light of order +1 or xe2x88x921 impinges upon the projection lens system and the contrast of the light-intensity distribution on the image plane can be improved. Since the contrast of the light-intensity distribution is thus enhanced by causing the illuminating light to impinge obliquely, satisfactory resolution can be obtained even if the position of the focal point is displaced. This makes it possible to increase depth of focus.
However, this modified illumination method is a technique that is effective in cases where the photomask pattern is a periodically repeating pattern that gives rise to diffracted light: it is not effective for an isolated pattern that lacks periodicity. Accordingly, for an isolated pattern of this kind, use is made of a method in which fine patterns (referred to as xe2x80x9cauxiliary patterns xe2x80x9d hereinafter in the entire disclosure) that are not allowed to be transferred to the semiconductor substrate are provided around the isolated pattern in order to provide this pattern with periodicity.
The technique using these auxiliary patterns will be described with reference to FIGS. 20 and 21. FIG. 20 illustrates a conventional photomask for the auxiliary pattern scheme, in which (a) is a view showing pattern layout on the photomask and (b) is a sectional view taken along line M-Mxe2x80x2 of (a). FIG. 21 is a graph illustrating the distribution of the intensity of light on the plane on which the image is formed in a case where exposure is performed using the photomask of FIG. 20.
With this conventional technique, in a case where use is made of the four-point illumination method, auxiliary patterns 2 are disposed above and below and to the left and right of the pattern to be resolved (referred to as main pattern 1) and are spaced a prescribed distance away from the main pattern 1 to provide the pattern with periodicity. If the position of the focal point is shifted in a case where only the isolated main pattern 1 is present, the skirt of the distribution of the intensity of light on the imaging surface broadens greatly and resolution undergoes a pronounced decline. When the auxiliary patterns 2 are provided, however, a phase-inverting effect ascribable to oblique incidence occurs between the main pattern 1 and the auxiliary patterns 2 and contrast is enhanced. Even if defocusing is performed, therefore, it is possible to suppress a decline in resolution.
Various problems have been encountered in the course of intense investigation towards the present invention.
Namely, by using the oblique incidence illumination method as the method of illumination and forming the auxiliary patterns in the photomask, the contrast of light intensity on the image plane is enhanced even at an isolated pattern and resolution can be improved. However, a problem which arises is that the auxiliary patterns themselves are transferred to the semiconductor substrate when the auxiliary patterns are formed on the photomask.
More specifically, because an auxiliary pattern has the main pattern on only one side thereof and no patterns on its other sides, the contrast of light intensity on the image plane does not become as large as it does in the case of the main pattern but the intensity of light at the portion corresponding to the auxiliary pattern is not zero either. Depending upon conditions such as the size of the main pattern, therefore, the auxiliary pattern also is transferred to the semiconductor substrate. This is discriminated as a defect in a wafer inspection (KLA, etc.).
Taking into account the effects of interference of transmitted light, the closer the size of an auxiliary pattern approaches that of the main pattern, the greater the contribution to contrast enhancement of the main pattern. If the auxiliary pattern is made too large, however, a problem which arises is that the auxiliary pattern itself tends to be transferred to the semiconductor substrate. Methods have been proposed to solve this problem. For example, there is a method of suppressing transfer of the auxiliary pattern by finely splitting the auxiliary patterns (see the specification of Japanese Patent Kokai Publication JP-A-10-92706), a method of lowering the contrast of the auxiliary patterns by shifting the position of the focal point in a number of stages (see the specification of Japanese Patent Kokai Publication JP-A-4-273428), and a method of forming the auxiliary pattern by engraving a photomask (see the specification of Japanese Patent Kokai Publication JPA-10-239827).
Among the methods mentioned above, the method of finely splitting the auxiliary patterns is effective in suppressing transfer of the individual auxiliary patterns. However, since portions substantially devoid of the auxiliary patterns are produced, a problem which arises is that contrast cannot be improved with regard to all portions of the main pattern.
The method of forming auxiliary patterns by engraving a photomask will be described with reference to FIG. 22. FIG. 22, which is described in a Japanese application filed previously by the present inventor, includes a view (a) showing the layout of patterns on a photomask and a sectional view (b) taken along line N-Nxe2x80x2 of (a).
According to this method, auxiliary patterns 2 disposed above and below and to the left and right of the main pattern 1 are formed by engraving a transparent substrate 4 in such a manner that phase of the auxiliary patterns shifts by 360xc2x0 with respect to the phase of the main pattern 1. With this structure, the phase of transmitted light is disturbed by the step or difference in level around the auxiliary patterns 2 so that interference is produced between this light and light that passes through the center of the auxiliary patterns. This has the effect of reducing the light intensity of the auxiliary patterns 2. However, it is still difficult to prevent satisfactorily the transfer of the auxiliary patterns themselves.
Accordingly, a principal object of the present invention is to provide a photomask and an exposure method using a photomask in which the contrast of light intensity of a pattern to be transferred is enhanced on an image plane while transfer of auxiliary pattern themselves is suppressed.
Other objects of the present invention will become apparent in the entire disclosure.
According to a first aspect of the present invention, there is provided a photomask comprising a desired main pattern to be transferred, and a plurality of auxiliary patterns which produce an interference effect with the main pattern by diffraction, wherein the plurality of auxiliary patterns are constituted by two or more pattern groups which have mutually different pattern sizes and which cause light that passes through respective ones of the auxiliary patterns to have mutually different phases on an image plane.
According to a second aspect of the present invention, there is provided a photomask comprising a desired main pattern to be transferred; a first auxiliary pattern disposed on at least one side of the main pattern at such a spacing that contrast of light, which passes through the main pattern, is enhanced on an image plane; and a second auxiliary pattern disposed on at least one side of the first auxiliary pattern at such a spacing that contrast of light, which passes through the first auxiliary pattern, is weakened on the image plane by diffraction.
According to a third aspect of the present invention, there is provided a photomask used in exposure to which is applied the modified illumination method for inverting the phase of light that passes through mutually adjacent patterns formed on a transparent substrate, comprising: four first auxiliary patterns disposed on a mask plane above and below and to the left and right of and a predetermined distance away from a desired main pattern to be transferred, thereby providing the transparent substrate with a surface flush with the main pattern or with an optical path difference corresponding to a phase difference, with respect to the main pattern, that is K times 360xc2x0 (where K is an integer that does not include zero); and four second auxiliary patterns disposed at positions corresponding to any of the sides above and below and to the left and right of the first auxiliary patterns at angles of 45xc2x0 with respect to the desired main pattern on the mask plane, thereby providing the transparent substrate with an optical path difference corresponding to a phase difference, with respect to the first auxiliary patterns, that is (2L+1) times 180xc2x0 (where L is an integer inclusive of zero).
According to a fourth aspect of the present invention, there is provided a photomask used in exposure to which is applied the modified illumination method for inverting the phase of light that passes through mutually adjacent patterns formed on a transparent substrate, wherein (a) the photomask has a plurality of main patterns to be transferred, the main patterns repeating at prescribed intervals; (b) four first auxiliary patterns are disposed on a mask plane above and below and to the left and right of and a predetermined distance away from one main pattern among the plurality of main patterns, thereby providing the transparent substrate with a surface flush with the one main pattern or with an optical path difference corresponding to a phase difference, with respect to the one main pattern, that is K times 360xc2x0 (where K is an integer that does not include zero); four second auxiliary patterns being disposed on the mask plane at positions corresponding to any of the sides above and below and to the left and right of the first auxiliary patterns at angles of 45xc2x0 with respect to the one main pattern, thereby providing the transparent substrate with an optical path difference corresponding to a phase difference, with respect to the first auxiliary patterns, that is (2L+1) times 180xc2x0 (where L is an integer inclusive of zero); the one main pattern, the four first auxiliary patterns and the four second auxiliary patterns constructing a first group; (c) another main pattern neighboring the one main pattern has a phase that is opposite that of the one main pattern; four second auxiliary patterns being disposed above and below and to the left and right of and a predetermined distance away from the other main pattern among the plurality of main patterns, thereby providing the transparent substrate with a surface flush with the other main pattern or with an optical path difference corresponding to a phase difference, with respect to the other main pattern, that is K times 360xc2x0 (where K is an integer that does not include zero); four first auxiliary patterns being disposed on the mask plane at positions corresponding to any of the sides above and below and to the left and right of the second auxiliary patterns at angles of 45xc2x0 with respect to the other main pattern, thereby providing the transparent substrate with an optical path difference corresponding to a phase difference, with respect to the second auxiliary patterns, that is (2L+1) times 180xc2x0 (where L is an integer inclusive of zero); the other main pattern, the four first auxiliary patterns and the four second auxiliary patterns constructing a second group; and (d) the first group and the second group are arrayed repeatedly so as to establish a relationship in which the auxiliary patterns constituting the first group are opposite in phase to the corresponding auxiliary patterns constituting the second group.
According to a fifth aspect of the present invention, there is provided a photomask used in exposure to which is applied a modified illumination method for inverting the phase of light that passes through mutually adjacent patterns formed in a transparent substrate, wherein the photomask has first line-shaped (linear) auxiliary patterns disposed on a mask plane on both sides of and a predetermined distance away from a desired line-shaped (linear) main pattern to be transferred, thereby providing the transparent substrate with a surface flush with the main pattern or with an optical path difference corresponding to a phase difference, with respect to the main pattern, that is K times 360xc2x0 (where K is an integer that does not include zero); and second line-shaped (linear) auxiliary patterns disposed on the mask plane outwardly of and a predetermined distance away from the first line-shaped auxiliary patterns, thereby providing the transparent substrate with an optical path difference corresponding to a phase difference, with respect to the first line-shaped auxiliary patterns, that is (2L+1) times 180xc2x0 (where L is an integer inclusive of zero).
According to a sixth aspect of the present invention, there is provided a multiple-stage focusing exposure method for performing exposure a plurality of times by achieving focusing on at least upper and lower levels of a difference in level when an image of exposing light is formed on a resist film that has been applied to a semiconductor substrate having the difference in level, characterized by performing exposure using the photomasks described above.
Other aspects and features are also mentioned in the appended claims, the disclosure whereof should be incorporated herein with reference thereto, upon needed.
By virtue of the construction described above, the present invention improves the contrast of the main pattern and, by bringing the phases of the auxiliary patterns into agreement, weakens the contrast of these patterns so that transfer of the auxiliary patterns can be suppressed and depth of focus enlarged.
Other features and advantages of the present invention will be apparent from the entire disclosure, particularly following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.